There is such a thing as too much vitamin C

You may hear each winter that vitamin C can fight off the common cold. But is it just hype or does it really help?

"The findings of a large number of studies on the subject are mixed," says Dr. Bruce Bistrian, chief of clinical nutrition at Beth Israel Deaconess Medical Center and professor of medicine at Harvard Medical School.

Vitamin C is a water-soluble essential nutrient found in highest concentration in fruits (particularly citrus), green vegetables and tomatoes. It's necessary for bone structure, iron absorption, skin integrity and immune function. A severe deficiency in vitamin C can cause a disease called scurvy, which is rare but can be fatal.

Because our bodies don't make vitamin C, we need to get it from external sources. The recommended dietary allowance for men is 90 milligrams (mg) per day, and for women it's 75 mg. At doses above 400 mg, virtually all administered vitamin C is excreted in the urine. There is limited concern about adverse effects up to 2,000 mg.

Above 2,000 mg, vitamin C can cause nausea, diarrhea and abdominal pain. Such high doses may interfere with the measurement of certain medical tests for blood and urine sugar (glucose) levels and blood in the stool, but most modern blood glucose testing methods are not affected.

Cold fighter?

Because vitamin C helps the immune system, researchers have spent decades investigating whether vitamin C can battle the common cold. The answer? It doesn't help if you begin taking it after cold symptoms start, and it doesn't reduce the number of colds you get if you take it regularly for prevention. However, if taken for prevention, it may help reduce the length of a cold.

"In 30 comparisons of nearly 10,000 respiratory episodes, there was an 8 percent reduction in a cold's duration in adults and a 13 percent reduction in children if vitamin C above 200 mg/day was taken preventively," Bistrian says.

Regular use of vitamin C may reduce the likelihood of developing a cold in certain people, however, Bistrian says. In people undergoing either extreme physical stress (such as marathoners) or physical stress plus cold stress (such as skiers and soldiers in military exercises in subarctic conditions), there was about a 50 percent reduction in risk.

What you should do

So what's the best way to get your daily recommended dose of vitamin C?

"The preferred mode of intake is food, although nearly 50 percent of Americans take a vitamin C supplement in some form," Bistrian says.

He points out that simply eating the recommended five servings of fruits and vegetables per day provides an intake of about 200 mg of vitamin C. If you're unable to obtain vitamin C from food, supplements can make up the difference.

Finding the C

The recommended dietary allowance for men is 90 milligrams (mg) per day, and for women it's 75 mg.